The following relates to the electrical power grid arts, electrical power grid frequency control arts, and related arts.
Electrical power grid management includes maintenance of the target electrical frequency, e.g. at 60 Hz (in North America) or 50 Hz (in Europe). Electrical power generators are designed to operate at (by way of illustrative example) 60 Hz at a given load. If the load increases, this creates counter-torque on the generators which slows the mechanical rotation and consequently lowers the electrical frequency. Conversely, if the load decreases, the counter-torque is reduced, the mechanical rotation speed increases and consequently the electrical frequency increases.
In practice, the electrical frequency is measured in real-time and is used for Automated Generation Control (AGC). The AGC compensates for two error components: (1) a frequency deviation from the target frequency (e.g. about 60 Hz in the U.S., or about 50 Hz in Europe); and (2) the mismatch between scheduled interchanges and actual interchanges. The mismatch information is used to balance out the imports/exports between two systems, e.g. every 10 minutes in some grids, so that one system is not “stealing” power from another. The interchanges are the connection points between any two systems. The AGC may employ throttling of ancillary generators (typically gas-fired for rapid response) up or down. Rather than ancillary generators, energy storage devices such as batteries or flywheels can also be used to absorb or inject power. The ancillary generators (and/or batteries, flywheels, et cetera) are property of the power company and/or property of curtailment service providers (CSPs). In the former case, construction and maintenance of these frequency control devices is a direct cost to the utility company or other grid operator. In the latter case, the grid operator typically contracts with the CSP to obtain access to the regulation resource for a prescribed time interval. In either case, scheduling of sufficient ancillary generator capacity typically done ahead of time, while the AGC is done using the ancillary generators, typically with a response time of minutes to tens of minutes for throttle-up or throttle-down of the ancillary generators.
The requirement to maintain sufficient ancillary generator capacity for Automatic Generation Control introduces substantial overhead cost and energy waste to the power grid. Overhead cost arises due to the need to construct and maintain the ancillary generators, and/or the cost of contracting with CSPs for access to these devices as well as inefficient fuel consumption by throttling generators. These costs can be reduced by improved aggregate load prediction or modeling; however, the grid operator still must plan for unexpected load swings due to weather changes, social events, renewable generation, unexpected industrial loads, and so forth. Energy waste arises due to operational inefficiencies of the ancillary generators (or batteries, flywheels, et cetera).
Automatic Generation Control (AGC) is also sometimes referred to as Secondary Frequency Control. In this context, Primary Frequency Control is provided in the form of frequency response at the load end. Loads providing frequency response are designed to detect the a.c. line frequency and to increase power usage (at least on average) when the frequency goes above the target frequency and to decrease power usage when the frequency goes below the target frequency. Frequency response operates locally to arrest frequency transients, but cannot correct frequency changes produced by such transients. Frequency response also cannot correct imbalances in energy exchanged between systems at interchanges, known as area control error (ACE). On the other hand, Secondary Frequency Control in the form of AGC can both ensure return to the target frequency after a transient and can correct interchange imbalances, i.e. Area Control Error (ACE).
An approach to reduce the need for ancillary generators or the like is Direct Load Control (DLC). In this approach, the grid operator directly controls load operation. By utilizing loads that have an energy storage and/or time lag facet, DLC can in principle provide a mechanism for Secondary Frequency Control. For example, heating, ventilation, and air conditioning (HVAC) systems have high thermal capacity due to the thermal capacity of the heated air volume. Similarly, hot water tanks have thermal capacity in the form of the stored hot water. Using DLC, the grid operator can operate such loads in a manner that provides Secondary Frequency Control while still maintaining the desired room temperature, water temperature, or so forth.